com.mawujun.util.other.WeakReferenceMonitor.java Source code

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Here is the source code for com.mawujun.util.other.WeakReferenceMonitor.java

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/*
 * Copyright 2002-2007 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.mawujun.util.other;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Track references to arbitrary objects using proxy and weak references. To
 * monitor a handle, one should call {@link #monitor(Object, ReleaseListener)},
 * with the given handle object usually being a holder that uses the target
 * object underneath, and the release listener performing cleanup of the
 * target object once the handle is not strongly referenced anymore.
 *
 * <p>When a given handle becomes weakly reachable, the specified listener
 * will be called by a background thread. This thread will only be started
 * lazily and will be stopped once no handles are registered for monitoring
 * anymore, to be restarted if further handles are added.
 *
 * <p>Thanks to Tomasz Wysocki for the suggestion and the original
 * implementation of this class!
 *
 * @author Colin Sampaleanu
 * @author Juergen Hoeller
 * @since 1.2
 * @see #monitor
 */
public class WeakReferenceMonitor {

    private static final Log logger = LogFactory.getLog(WeakReferenceMonitor.class);

    // Queue receiving reachability events
    private static final ReferenceQueue handleQueue = new ReferenceQueue();

    // All tracked entries (WeakReference => ReleaseListener)
    private static final Map trackedEntries = Collections.synchronizedMap(new HashMap());

    // Thread polling handleQueue, lazy initialized
    private static Thread monitoringThread = null;

    /**
     * Start to monitor given handle object for becoming weakly reachable.
     * When the handle isn't used anymore, the given listener will be called.
     * @param handle the object that will be monitored
     * @param listener the listener that will be called upon release of the handle
     */
    public static void monitor(Object handle, ReleaseListener listener) {
        if (logger.isDebugEnabled()) {
            logger.debug("Monitoring handle [" + handle + "] with release listener [" + listener + "]");
        }

        // Make weak reference to this handle, so we can say when
        // handle is not used any more by polling on handleQueue.
        WeakReference weakRef = new WeakReference(handle, handleQueue);

        // Add monitored entry to internal map of all monitored entries.
        addEntry(weakRef, listener);
    }

    /**
     * Add entry to internal map of tracked entries.
     * Internal monitoring thread is started if not already running.
     * @param ref reference to tracked handle
     * @param entry the associated entry
     */
    private static void addEntry(Reference ref, ReleaseListener entry) {
        // Add entry, the key is given reference.
        trackedEntries.put(ref, entry);

        // Start monitoring thread lazily.
        synchronized (WeakReferenceMonitor.class) {
            if (!isMonitoringThreadRunning()) {
                monitoringThread = new Thread(new MonitoringProcess(), WeakReferenceMonitor.class.getName());
                monitoringThread.setDaemon(true);
                monitoringThread.start();
            }
        }
    }

    /**
     * Remove entry from internal map of tracked entries.
     * @param reference the reference that should be removed
     * @return entry object associated with given reference
     */
    private static ReleaseListener removeEntry(Reference reference) {
        return (ReleaseListener) trackedEntries.remove(reference);
    }

    /**
     * Check if monitoring thread is currently running.
     */
    private static boolean isMonitoringThreadRunning() {
        synchronized (WeakReferenceMonitor.class) {
            return (monitoringThread != null);
        }
    }

    /**
     * Thread implementation that performs the actual monitoring.
     */
    private static class MonitoringProcess implements Runnable {

        public void run() {
            logger.debug("Starting reference monitor thread");
            try {
                // Check if there are any tracked entries left.
                while (!trackedEntries.isEmpty()) {
                    try {
                        Reference reference = handleQueue.remove();
                        // Stop tracking this reference.
                        ReleaseListener entry = removeEntry(reference);
                        if (entry != null) {
                            // Invoke listener callback.
                            entry.released();
                        }
                    } catch (InterruptedException ex) {
                        logger.debug("Reference monitor thread interrupted", ex);
                        break;
                    }
                }
            } finally {
                logger.debug("Stopping reference monitor thread");
                synchronized (WeakReferenceMonitor.class) {
                    monitoringThread = null;
                }
            }
        }
    }

    /**
     * Listener that is notified when the handle is being released.
     * To be implemented by users of this reference monitor.
     */
    public static interface ReleaseListener {

        /**
         * This callback method is invoked once the associated handle has been released,
         * i.e. once there are no monitored strong references to the handle anymore.
         */
        void released();
    }

}